A study on performance trends on Google's Content Network compared the cost per acquisition (CPA) on Google's Content and Search Networks and found that, for the median advertiser, the average CPAs were about the same on both channels. Google made some clarifications to the executive summary and concluding paragraph of the white paper, after receiving questions from advertisers about why CPAs were similar and what actions you should take based on the findings in the white paper.

More info: AdWords blog